Tuesday, January 27, 2009

Triceratops Combat?

Exactly how did Triceratops and other horned dinosaurs use their cranial weaponry? Today, my co-authors Ewan Wolff, Darren Tanke, and I published new research in the online, open access journal PLoS ONE, giving our take on the issue. In a study spanning four years and over a dozen museums, we have marshalled what we think is the best evidence to date that Triceratops may have locked horns with their own kind.

The BackgroundThe ceratopsids, or horned dinosaurs, present a bewildering array of cranial headgear. Triceratops was one of the earliest discovered and best known, with its distinctive dual brow horns, single nose horn, and a solid frill of bone over the neck. Subsequent discoveries revealed a clade chock full of anatomical diversity - single horns, triple horns, and frills sporting all sorts of odd accessories. Consequently, a whole host of functional explanations have been dreamed up for these bones.

Defense against predators was an early, popular explanation. With threats such as Tyrannosaurus stalking the Cretaceous landscape, Triceratops and relatives surely would have used their horns for defense if they had to. Yet, this is not the primary function of the horns in modern animals. More commonly, horns, antlers, and other appendages are used for intraspecific combat and display--not against predators. A good example is the massive horns of the bighorn sheep. Careful behavioral studies have shown them to be employed in impressive visual display to other sheep, or for combat over territory, mating rights, dominance, etc. These modern analogs were quickly compared with ceratopsids.

In addition to the determination of function by analogy, some paleontologists noted odd markings on the skulls of some horned dinosaurs. Perhaps a chunk of the frill was missing, with evidence of healing. Maybe there was an abnormal, oddly positioned hole on the skull. Inevitably, these oddities were attributed to "horn gouge wounds." These alleged pathologies were interesting--but anecdotal. Without a broader context, any attempts to infer a cause for the "injuries" usually amounted to constructing a "Just So Story."

Recently, the tide has turned against the idea of horns-as-weapons in Triceratops and its relatives. Research by Kevin Padian, Jack Horner, Mark Goodwin, and others have suggested that the odd cranial ornamentation of many dinosaurs was for more peaceful purposes--specifically, species recognition (picking out members of their own kind amongst a sea of similar species). Although this idea certainly has its merits, I was never quite convinced that the horns of Triceratops were completely benign (and to be fair, I don't know that the species recognition proponents were suggesting this as the only function, either). At the same time, I wasn't exactly convinced that every single ding on a fossil skull should be considered evidence for combat.

What We DidI've been really interested in the issue of horned combat in Triceratops for some time. Several years ago, I published a brief study, using plastic models, that demonstrated such combat was at least physically possible. I made some predictions about where we should expect injuries on the skull, noted similarities with published anecdotes of alleged injuries, and left the issue open for a follow-up study. The models were fun to play with, but I wanted to know if this combat actually happened in real life! And, I knew that paleopathology--the study of disease and injury in fossils animals--was probably the best approach. Unfortunately, I didn't know that much about the topic.

Our logic was simple--if ceratopsid dinosaurs used their horns against each other in combat, evidence of this should be found in bony lesions on the skull. And, assuming that animals with different horn configurations fight differently (a fair assumption from observations of modern horned critters), we should find differing rates and patterns of lesions between different species of horned dinosaurs. If the horns were not used in combat (or specifically, patterns of horn locking that could result in injury to the skull), we should see no difference between various species.

So, we surveyed many, many specimens at over a dozen museums across North America. We looked for any evidence of pathology, and recorded its location on the skull. Two genera were the focus of our research--Triceratops (pictured at the top of the post) and Centrosaurus (pictured below this paragraph). Centrosaurus has only a single nasal horn (and no massive brow horns), so we assumed that it would be most likely to show differences from Triceratops if there were any to be found. After years of data collection, we subjected all of the numbers to statistical analysis. Lots of folks (Darren and I included) had described anecodotal reports of pathology before--but nobody had subjected them to the rigors of statistics.

What We FoundBriefly, we found that most bones of the skull that we examined did not show statistically significant differences (i.e., P>0.05) in rate of pathology between the two genera. Except, that is, for the squamosal bone of the frill (shown on the figure below). But what does this mean?

Schematic showing rates of pathology (number of pathological elements/total elements examined) for Triceratops (top) and Centrosaurus (bottom). The squamosal rates, highlighted here, were different from each other at P<0.05.

Did Triceratops Fight Each Other?As you can probably guess by now, we postulate that the differences in rates of pathology between Triceratops and Centrosaurus were caused by behavioral differences. In other words, because Triceratops were locking horns with either other, they got injured. However Centrosaurus used its horns (whether for combat or display, or whatever), they didn't commonly cause injury to the squamosal bone. We do consider some alternative explanations for the lesions (predator attack, microbial origin, etc.), but generally discount these (read the paper for a full explanation).

As a reminder, if the horns were only being used for display (species recognition, or whatever), we wouldn't expect to have seen these differences in pathology rates between the two species. Thus, I am reasonably comfortable saying that the horns weren't "just for looks."

What Does This Mean for Dinosaur Research?The statistical study presented here is some of the best evidence to date (in our opinion) for intraspecific combat in Triceratops. Centrosaurus may well have done the same, just in a very different manner (flank butting, or something like that). Or, maybe Centrosaurus was a more peaceful taxon--we just don't know yet. Lots more research is needed in this area.

Another big question is why these animals were fighting. Mating rights? Sexual dimorphism hasn't been demonstrated in the taxon, so it doesn't seem like males and females are set up any differently in terms of horn configuration (but further study may change this). Territory? Who knows (and good luck investigating this with fossils!). General bad temper? Your guess is as good as mine. We may never know the why in this case.

Centrosaurus came from an ancestor with big old brow horns--perhaps this ancestor lost the horns in favor of a kinder, gentler approach. The latest centrosaurines (living millions of years after Centrosaurus), such as Pachyrhinosaurus, ditched their horns altogether in favor of big bony pads on the skull. So, did centrosaurines evolve towards less risky forms of combat? Again, this is another hypothesis that should be tested more thoroughly. There's lots more work to do!

On a more methodological note, we hope that more studies of dinosaurian paleopathology will adopt a statistical approach. Anecdotes are interesting, and can be very informative in some cases, but they only tell part of the story. There is strength in numbers!

What Aren't We Saying?Because cranial function is a contentious issue, I just want to make clear on what we aren't claiming. (I am speaking for myself here--my co-authors are probably largely in agreement)

All injuries on ceratopsid skulls are attributable to combat. No, no, no. Just because there is a ding on the skull doesn't mean it's a combat wound. Almost certainly, some non-combat injuries are included in our sample--this is why we looked at overall patterns, rather than anecdotal cases. I would be willing to say, though, that the overall pattern of lesions (at least on the squamosal) suggests that most of them were caused by the horns of other ceratopids.

Ceratopsid horns were only for combat. Again, no. This isn't the case in modern animals, and it likely wasn't the case for ceratopsids. As I (and Darren) have said elsewhere, ceratopsid horns were probably like Swiss Army Knives--multi-purpose tools. Fighting, defending, showing off, cooling off, you name it. I am just saying that now we shouldn't exclude fighting as a function.

All ceratopsids used their horns identically. No. Different horn morphologies suggest different uses--and I would be willing to believe (as others have suggested) that animals such as Pachyrhinosaurusfocused on flank-butting behavior (rather than head-to-head combat). Again, this will need more work.

Parting ThoughtsThis has been a really, really fun study, and one that I hope will spur more research on the possibilities for skull function in horned dinosaurs. We haven't heard the last on this issue, rest assured. There are lots more specimens, new species to describe and discover, and many other people working to better understand these magnificant animals. I would also like to publicly thank my co-authors--I have learned so much about paleopathology and ceratopsids from you! Finally, a big thank you to our artist, Lukas Panzarin, who skillfully brought to life a long-vanished Triceratops battle.

21 comments:

Makes sense. But the shape of Triceratops' brow horns could be used for predator defense couldn't they? Couldn't they be used to threaten a predator like Tyrannosaurus rex?

However, I'm NOT saying I disagree with the dominance, territorial, display, etc. behavior that we see in mountain goats, stags, etc. today. That does seem more likely than predator defense. But, then, why Triceratops need such a thick frill?

@Raptor Lewis - absolutely. I think the horns were probably used however they were needed (and predator defense would have been among those uses). As for the thick frill in Triceratops, that's a bit of a mystery. . .

@ReBecca - thanks! Hope it doesn't put you to sleep. . .

@Jordan - thanks! It was one of those projects I always wanted to do, but needed several years of data collection (and competent co-authors) to make it happen!

I've been very interested in any follow-up research since your Pal Electronica study on model Triceratops skulls. I'm really excited to see it in print. I agree with you on stats; I try to apply stats when possible.

Aaaah. Well done, sir. I printed and read this paper awhile back because I get early notice of these papers. I forgot about the embargo, though, so I didn't realize you weren't able to talk about it yet!

But yeah, wonderful paper! Good to see a conceptual idea vindicated with fossil evidence. Congrats, brother. I'd like to see an investigation into how pachyrhinosaurs used THEIR unique "weapons" in the future...you do take requests, right? ;-)

Nice that behavior something we've long suspected in trikes has finally been confirmed.

I think the nose horns of centrosaurines would be better suited to defense against tyrannosaurids than those of chasmosaurines like Triceratops. Centrosaurine nose horns are generally much larger than those of chasmosaurines, for one.

Quick question for you, Andy: How do you know your results reflect behavioural differences between the two ceratopsid genera, rather than behavioural differences between the two tyrannosaurid genera that supposedly fed on them?

Oh, you would ask that, wouldn't you, Jordan?! I don't know that we'll ever know for sure. . .I guess I would maybe predict different kinds of injuries for tyrannosaurs (bigger ones, maybe?). Who knows, maybe there are a few "Rex Bites" in our sample. . .

you'd think the parietal portion of the frill would get hit a lot, if triceratops were locking horns and fighting each other, wouldn't you?

There would be lots of twisting of heads and the horns would slip up to the parietal region of the frill, leaving pits, gouges etc.

But yet the research shows very little evidence of injuries in this area. So to me, the observations don't support dueling triceratops.

I should mention that i suspect strongly that triceratops did fight each other. Most large animals today do (although they are mammals, which may have very different behaviours from dinosaurs), so it seems reasonable and likely that prehistoric large animals also did (but again it could be a mammal thing).

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This is a personal weblog, and the opinions expressed here represent my own and not those of my employer or any other organization with which I am associated. Although I strive for accurate information, please be aware that the advice offered here is "as is," with no claims or guarantees that it will work for you. Hey, I'm just a paleontologist.